Like I said, if they think Disney is going to make 50M off of Spidey, and Sony's only getting 20M for licencing him out, they're not going to go for it. For a businessperson, that's like giving away free money. Business-wise it'd be better to just keep the character so that Sony's the only place you can get Spidey on film, keep the demand high, and use the fact that the public hasn't seen Spidey since the last Spidey movie to make 50M.

I imagine that Sony would want to get in on that Avengers hype/money. I see no reason why they wouldn't let Marvel use him in a movie. Also, there's the possibility that including him could boost the marketability of the Amazing Spider-Man film series. If subsequent Marvel films get a big bump off The Avengers, the math will start to make more sense for Disney and Sony to make a deal. Everyone stands to benefit from it.